The most common absorbent for carbon dioxide is hydrated lime, or calcium hydroxide. Water is required in the absorbent formulation to serve as a substrate in the net CO2 absorption reaction:CO2(g)+Ca(OH)2(s)→CaCO3(s)+H2O(g,l),in which carbon dioxide reacts with lime to produce calcium carbonate and water. Traditionally, the lime is combined with sodium and/or potassium hydroxide to form soda lime. The presence of strong alkali hydroxides allows soda lime to absorb carbon dioxide more quickly and with greater capacity than mixtures containing only lime and water.
A disadvantage of soda lime is the sensitivity of anesthetic agents to strong hydroxide bases. The most commonly used modern anesthetic agents are fluorinated hydrocarbons, which may chemically react with sodium and/or potassium hydroxide to form potentially toxic by-products. For example, the anesthetic sevoflurane (1,1,1,3,3,3-hexafluoro-2-(fluoromethoxy)propane) has been found to undergo hydroxide-induced dehydrofluorination to produce a fluoro-olefin byproduct called “Compound A” (fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether), which is nephrotoxic to rats at concentrations of 60 to 100 ppm and lethal at concentrations of 350 to 400 ppm.
An additional concern when using absorbents containing sodium and/or potassium hydroxides is the possibility of desiccation of the mixture. Highly exothermic chemical reactions between anesthetic agents and dry soda lime release formaldehyde, methanol, and carbon monoxide, which are undesirable by-products. In extreme cases, the heat liberated by these chemical reactions causes a fire.
Two main approaches have been taken to address the problem of by-product Compound A formation and exothermic degradation to formaldehyde, methanol, and carbon monoxide. The first approach is to remove strong alkali hydroxides, such as sodium and potassium hydroxide, from the absorbent formulation. U.S. Pat. No. 6,228,150 and U.S. Pub. No. 2004/0029730 each disclose a carbon dioxide absorbent comprising calcium hydroxide essentially free of sodium and potassium hydroxide. World publication WO 01/45837 discloses a carbon dioxide absorbent comprising lime essentially free of sodium hydroxide, essentially free of water, essentially free of a humectant, and containing at least 20% anhydrous lithium hydroxide. German patent publication DE 197 40 736 discloses the use of lime free of sodium hydroxide and potassium hydroxide, to which barium hydroxide and/or magnesium hydroxide and/or lithium hydroxide may be added. No ranges are disclosed in DE 197 40 736, nor is the water content of the formulation. None of the publications listed above disclose the use of 0.1 to 17% lithium hydroxide, any of its precursors, or any combination thereof, with 70 to 90 wt % hydrated lime and 5 to 25 wt % water.
The second approach used to circumvent anesthetic degradation is the addition of a hygroscopic salt or other humectant to enhance the water retention and/or color indicating properties of the absorbent mixture. Canadian patent 1151633 discloses the use of calcium chloride to enhance the water retention properties of soda lime. U.S. Pub. No. 2004/0029730 discloses the use of 0.2 to 2.0 wt % calcium chloride and/or magnesium chloride in a soda lime absorbent formulation. U.S. Pat. No. 6,228,150 discloses a lime-based carbon dioxide formulation which includes an inorganic humectant, wherein the humectant is calcium chloride hexahydrate and/or magnesium chloride hexahydrate, present in an amount sufficient to yield 7.5 to 20 wt % water. In addition, this publication discloses the addition of 2.5 to 25 vol % of glycerol as an organic humectant. Glycerol is also known to be reactive toward olefin products such as Compound A (Cunningham et al. 1996). U.S. Pat. No. 6,867,165 discloses a carbon dioxide absorbent comprising calcium hydroxide, which may contain sodium or potassium hydroxide, water, a rheology modifier taken from a group of phosphonic acids and salts, and 0.1 to 6.0 wt % calcium chloride to improve color indication properties. None of the publications listed above disclose the use of calcium chloride or any other humectant in a formulation comprising 0.1 to 17 wt % lithium hydroxide or any of its precursors, or any combination thereof, with 70 to 90 wt % hydrated lime and 5 to 25 wt % water.